S11-B is a permanent line of human lymphocytes transformed in vitro with E-B virus (by S. Shaw, IB, NCI). ACF-9, a subclone of this line, express mIgM, mIgA, HLA, and HLA-DR antigens, and FcR(Hu). This line responds to a lymphokine(s) in human MLR culture fluids with (a) slowed growth, or death at high concentrations, and (b) with a concomitant tenfold increase in HLA-DR antigens, as judged by flow cytometry. There was no change in expression of HLA antigens, FcR, IgA or IgM in induced cells. The MLR culture fluids produced the change in HLA-DR at high dilution (e.g. 5x10 minus 2). Higher concentrations were needed to slow cell growth. Two human-mouse hybridomas, produced by immunizing human cells in vitro with tetanus toxoid, then fusing the stimulated cells with a mouse myeloma cell were found to be double secretors: each hybridoma, and more than 30 subclones of each secreted both IgM and IgG in high titer. Every cell in each clone stained with fluorescent labeled antibodies to human IgG and IgM. Elisa tests showed that each clone made both human IgG and IgM antibody to tetanus toxoid. These hybrids are the first examples of "biclonal" lines in which each immunoglobulin has antibody activity to the same antigen. These lines may be useful in the study of the "isotype switch".